1. Field of the Invention
The present invention relates in general to receivers for decoding 100Base-TX signals, and in particular to a receiver employing a finite impulse response (FIR) filter for adaptive equalization and automatic gain control.
2. Description of Related Art
The IEEE 802.3 (xe2x80x9cEthernetxe2x80x9d) standard defining a digital media interface commonly used for transmitting data between computers linked through a network includes a xe2x80x9c100Base-TXxe2x80x9d protocol for category 5 (CAT5) twisted-pair data transmission employing MLT-3 line encoding. FIG. 1 is a timing diagram illustrating how data is represented by an xe2x80x9cidealxe2x80x9d MLT-3 encoded waveform. The MLT-3 waveform is organized into a sequence of 8 ns (nanosecond) data cycles and conveys data at a rate of 125 Mb/s (megabits per second), one bit of data per data cycle. The MLT-3 standard defines three voltage levels referenced as +1, 0 and xe2x88x921. During each cycle the waveform may stay at the same level, or may transition from level +1 to level 0, from level 0 to level xe2x88x921, from level xe2x88x921 to level 0, or from level 0 to level +1. When the waveform does not transition between levels during a current (Kth) data cycle, it represents the same data value as it represented during the preceding (Kxe2x88x921)th data cycle. When the waveform transitions during the Kth data cycle, it represents a bit of state opposite to that of bit represented during the preceding (Kxe2x88x921)th data cycle.
As the MLT-3 waveform travels over a network it can be distorted in ways that can make it difficult for a receiver to extract the data conveyed by the waveform. Communication channels conveying the MLT-3 waveform include magnetic modules coupling the waveform onto the twisted pair cable. Since these modules act like high pass filters, they attenuate low frequency components of the waveform. Thus when an MLT-3 waveform has relatively few transitions during a relatively long period, it appears as a low frequency signal, and the magnetic modules cause the waveform to suffer a type of distortion called xe2x80x9cbaseline wanderxe2x80x9d as illustrated in FIG. 2. The twisted pair conveying an MLT-3 waveform also has an insertion loss and a frequency-dependent attenuation that can further distort the waveform, for example as illustrated in FIG. 3. To avoid errors in decoding an MLT-3 waveform, a 100Base-TX receiver should provide baseline wander correction, gain control compensation for insertion loss and equalization for frequency-dependent attenuation of the twisted pair.
The paper entitled xe2x80x9cA CMOS Transceiver for 10 Mb/s and 100 Mb/s Ethernetxe2x80x9d, by Everitt et al, published December 1998 in the IEEE Journal of Solid-State Circuits, Vol 33, No 12, describes a receiver illustrated herein in FIG. 4. An attenuator 12 attenuates the input MLT-3 signal to prevent clipping. A summer 14 adds the output of a baseline compensation circuit is to the output of attenuator 12 to level shift the MLT-3 signal as necessary to compensate for any base line wander. A high-pass filter 18 having adjustable frequency response characteristics filters the output of summer 14 to provide equalization, and an automatic gain control (AGC) circuit 20 amplifies the output of summer 14 to compensate for insertion loss. A summer 22 sums the outputs of high-pass filter is and AGC 20 to supply an equalized and compensated MLT-3 signal to a low-pass filter 24 which filters high frequency noise out of the signal.
A three-level clock-recovery slice circuit 26, phase-locked loop circuit 28 and 180 degree phase shifter 30 process the waveform output of low pass filter 24 to produce a 125 Mhz clock signal (CLOCK) phase locked to the MLT-3 waveform. The CLOCK signal clocks a six-level bit slice circuit 32 which produces data representing the voltage of the MLT-3 waveform output of low pass filter 24 as one of six levels. Other circuits (not shown) recover the transmitted data from the output data of slice circuit 32. The data output of slice circuit 32 also provides input to a digital logic circuit 34 which processes that data to control the amount compensation to be provided by baseline wander compensation circuit 16 and AGC circuit 20 and to adjust high pass filter 18 as needed to compensate for frequency-dependent attenuation.
With a high component count, the receiver requires substantial power and die area. What is needed is a 100Base-TX receiver having lower power and die area requirements.
A 100Base-TX Receiver in accordance with the invention employs a finite impulse response (FIR) filter to provide both equalization and insertion loss compensation for an MLT-3 input signal.
The FIR filter includes three delay stages, each delaying the input signal with an 8 ns delay (the period of one data cycle of the MLT-3 input signal), a set of three amplifiers for amplifying the delay stage outputs with gains C1, C2 and C3, and a summer for summing the outputs of the three amplifiers to produce a compensated, equalized MLT-3 signal. The receiver also includes a low-pass filter for filtering high frequency noise out of the FIR filter output signal.
A data slicer digitizes the low-pass filter""s output MLT-3 signal during each data cycle to produce slice data representing that MLT-3 signal as being within one of six levels. An adaptive control signal processes the slice data to adaptively adjust the gains C1, C2 and C3 of the three FIR amplifiers to provide a correct amount of equalization and insertion loss compensation.
The adaptive control circuit also processes the slice data to adaptively adjust the phase of a clock signal controlling timing of the data slicer, to adaptively adjust an amount of baseline wander compensation provided to the MLT-3 input signal, and to produce an output digital signal representing the data sequence conveyed by the MLT-3 input signal.
The use of an FIR filter to provide equalization and compensation reduces the amount of area on an integrated circuit die needed to implement a 100Base-TX receiver and allows the receiver to operate with less power than prior art 100Base-TX receivers.
The concluding portion of this specification particularly points out and distinctly claims the subject matter of the present invention. However those skilled in the art will best understand both the organization and method of operation of the invention, together with further advantages and objects thereof, by reading the remaining portions of the specification in view of the accompanying drawing(s) wherein like reference characters refer to like elements.